Device for dynamic tensioning of a natural or prosthetic knee joint

ABSTRACT

The device includes at least one femoral insert ( 8 A) with a condyle support surface ( 20 A), for a bone or femoral implant, at least one tibial insert ( 10 A), with a tibial plateau support surface ( 24 A), for a bone or a tibial implant and force introduction element ( 4 A,  30 A), for application between the femoral insert and the tibial insert of a distraction force of a given intensity with the knee cap in place or not. The condyle support surface ( 20 A) is dished in form and is provided with a sliding element ( 12 A) for the bone or femoral implant when the knee joint is moved.

The present invention relates to a device for dynamic tensioning of aknee joint, also referred to as a device for distraction of the knee.

In the field of arthroplasty of the knee, the surgeon seeks to replacethe damaged or deficient natural joint of the knee with a prostheticjoint which reproduces as faithfully as possible the kinematicproperties of natural joints whilst forming a stable, durable andpainless structure. The soft parts (capsule, ligaments and tendons) ofthe knee joint play a significant role in the mechanical strength of thejoint when it is caused to move. However, these soft parts are specificto each patient and may be affected to a greater or lesser extent, forexample, following illnesses. Furthermore, when prosthetic knee implantsare positioned, the surgeon is often caused to excise specificligaments, creating a new biomechanical environment.

It is therefore necessary, when a knee prosthesis is implanted, toevaluate the tension of these soft parts and, if necessary, to correctit in order to provide the best possible positioning for the prosthesis.More precisely, the desired object is to obtain tensions of the softparts of the knee which are equal at 0 and 90° of flexion and which aremaintained over the entire flexion arc of the prosthesis, satisfactorygeometric alignment and extension without flexum in order to optimisethe stresses in the upright position and provide the best possible fitwith respect to the anatomy of the patient. A significant object is toobtain a good level of stability for the knee by means of an appropriateligament balance.

To this end, a device for tensioning the soft parts is generally used,commonly referred to as “tensor” which generally comprises a femoralinsert having two condyle support surfaces for the femur, a tibialinsert having at least one support surface for the tibial plate, andmeans for applying, between the femoral and tibial inserts, adistraction force of a predetermined strength. Known means for measuringthe relative positions of the femur and the tibia are associated withthis tensor so that, by introducing the tensor in the space between thetibial end and the femoral articular end, it is possible to determine,under the selected tension value imposed by the tensor, the spacingbetween the tibia and the femur, as well as the angle HKA, that is tosay, the angle taken internally between the femoral mechanical axis(defined by the centre of the hip and the centre of the knee) and thetibial mechanical axis (defined by the centre of the knee and the centreof the ankle), on the one hand, for extension and, on the other hand, ina state of flexion at 90°. Based on the measurements carried out in thismanner, the surgeon selects the most appropriate constituent elements ofthe prosthesis, in particular from the set of elements which he has athis disposal.

However, it has been found that the use of tensors of this type does notalways ensure optimum selection and/or positioning of the selectedprosthetic elements, which does not allow optimum biomechanics to beachieved, in particular during retraction of the posterior soft parts ofthe knee in a state of flexum, and in the intermediate phases of flexionbetween 0 and 90°, and beyond 100°. The optimum biomechanics correspondto a “good tension” of the soft parts in all sectors of movement, thatis to say, stability tension for the support zones and a micro level offrontal and rotational laxity clearance between 20 and 140°, allowingease of movement without ever having hypertension or excessive orunequal laxity.

The object of the present invention is to provide a tensioning devicewhich overcomes the above-mentioned disadvantages by allowing continuouscontrol, that is to say, over substantially the entire flexion path ofthe knee joint, of the “good tension” of the soft parts.

To this end, the invention relates to a device for dynamic tensioning ofa natural or prosthetic knee joint, of the type comprising a tensioningdevice for a natural or prosthetic knee joint, with or without a tibialcut being carried out, of the type comprising at least one femoralinsert which has a condyle support surface for a femoral implant orbone; at least one tibial insert which has a support surface for atibial plate for a tibial implant or bone; and means for applying,between the femoral and tibial inserts, a distraction force of apredetermined strength, with or without the kneecap being in position,characterised in that it is constructed so as to allow rotation of thejoint and comprises means for maintaining the knee in a state of tensionduring rotation, and thus carrying out the measurements for variousangles of rotation.

According to other features of this device, taken in isolation oraccording to all technically possible combinations:

-   -   the condyle support surface is in the form of a dish and is        provided with sliding means for the femoral implant or bone when        the knee joint is displaced;    -   the sliding means comprise juxtaposed rollers;    -   the sliding means comprise juxtaposed ball-bearings;    -   the condyle support surface is substantially cylindrical, having        an axis which is substantially transverse relative to the        direction-of distraction;    -   the maximum thickness of each femoral insert and tibial insert        is less than or equal to 2.5 mm;    -   a femoral insert, and optionally a tibial insert, is/are        provided for each inner and outer compartment of the knee joint;    -   it comprises means for measuring the spacing of the condyle        support surfaces and tibial plate support surfaces, which means        are capable of continuously measuring the spacing between the        support surfaces when the knee joint is displaced;    -   it comprises means for measuring the distraction force between        the femoral inserts and tibial inserts, which means are capable        of continuously measuring the variation of the strength of the        distraction force around the predetermined strength thereof when        the knee joint is displaced;    -   the means for applying the distraction force comprise a force        generation unit and a pair of branches which connect the        generation unit to the femoral inserts and tibial inserts.

The invention will be better understood from a reading of the followingdescription, given purely by way of example and with reference to thedrawings, in which:

FIG. 1 is a perspective view of a tensioning device according to theinvention,

FIG. 2 is a section through plane II-II indicated in FIG. 1;

FIG. 3 is a schematic front view of a natural knee joint, at the innerside of which the femoral and tibial inserts of the device of FIG. 1,illustrated in section, are placed; and

FIG. 4 is a schematic lateral view corresponding to FIG. 3.

FIG. 1 illustrates a device 1 for tensioning a knee joint. This device 1is substantially formed by two similar assemblies, that is to say, aninner assembly 2A for the inner compartment of the joint in the regionof the inner femoral condyle, and an outer assembly 2B for the outercompartment (in the region of the outer femoral condyle). Forconvenience, in all of the following description, the device will bedescribed and orientated with reference to a standard knee joint, theterms upper or top, lower or bottom, anterior or front, and posterior orrear, and the terms inner and outer corresponding to those used tocommonly describe a joint of this type.

Furthermore, since the inner assembly 2A and outer assembly 2B comprisethe same elements, only the elements of the inner assembly 2A will bedescribed below, the corresponding elements of the outer assembly 2Bbeing designated by the same number, followed by the letter B.

The inner assembly 2A is generally in the form of a pincer and comprisestwo branches 4A which are articulated relative to each other about apivot axis 6A. The distal ends of the branches 4A are provided,respectively, with inserts (metal or non-metal), that is to say, afemoral insert 8A which is intended to be placed in contact with a lowerend portion of the femur, in particular the condyle, and a tibial insert10A which is intended to be placed in contact with an upper end portionof the tibia. The femoral and tibial inserts can be moved relative toeach other and are particularly suitable for moving away from each otherin accordance with a trajectory which is substantially in the form of anarc of a circle and which is centred on the articulation axis 6A whenthe proximal portions of the branches 4A are brought together.

More precisely, as illustrated in FIG. 2, the femoral insert 8A isgenerally in the form of a cylinder segment having an axis X-X whichextends in a transverse direction. It comprises a series of juxtaposedrollers 12A which are mounted so as to rotate freely about axes 14A andwhich are, for example, fixedly joined to a common plate 18A which isrigidly connected to the corresponding branch 4A. The axes 14A extendsubstantially parallel with the axis X-X. The insert 8A thus provides aconcave upper surface 20A in the form of a dish. This surface 20A isintended to form a support for the inner condyle of the femur, theradius of curvature of the dish 20A being selected so as to be close tothe mean radius of curvature of this inner condyle in the sagittalplane. Of course, the rollers may be replaced with a fixed supportsurface.

The tibial insert 10A itself comprises a plate 22A having outerdimensions which are substantially similar to those of the femoralinsert 8A. The plate 22A extends in a direction substantially parallelwith the axis X-X of the femoral insert 8A. The tibial insert has, atthe lower side, a surface 24A for supporting an inner tibial plate, thatis to say, the natural inner upper surface of the upper end of the tibiarelative to the inner condyle of the femur, or a substantially planarsurface which is provided in this end of the tibia, for example, bymeans of a saw.

The femoral inserts 8A, 8B and tibial inserts 10A, 10B are thin, forexample, in the order of 2.5 mm each, in order to be able to be slidbetween the femur and the tibia, with the knee cap advantageously notbeing in a dislocated state as illustrated in FIG. 3, in the region ofeach inner and outer compartment of the knee joint before the femur iscut. The inserts can be removable relative to the branches 4A and 4B andare fixed to these branches using rapid fixing means. The inserts arethus readily positioned in the condyle compartments whilst they are notyet connected to the branches of the device.

The proximal ends of the branches 4A of the inner assembly 2A areconnected to each other by means of a unit 30A for generating a forcewhich tends to bring together these ends. More precisely, the generationunit 30A comprises a piston 32A which is rigidly connected to one of thebranches 4A, and a cylinder 34A which is rigidly connected to the otherof the two branches 4A and at the inner side of which the piston 32A canmove. The upper end of the cylinder 34A is provided with a screw 36A forsealed closure, delimiting, with the inner walls of the cylinder and thehead of the piston 32A, a chamber 38A of variable volume. A pressurisedfluid source, provided with control means which are not illustrated, isconnected to this chamber 38A, via a connector 40A which is providedwith a pressure gauge 42A, and control means 44A, 46A. This motorfunction carried out by this piston and this fluid may, in a variant, becarried out by an electric servomotor.

Advantageously, the device 1 comprises means which are not illustratedfor measuring the spacing of the support surfaces 20A, 20B, 24A and 24B.These means, which are well known in the surgical field, comprise, forexample, a high definition digital camera which is associated with aninfrared emission source which covers the range in which a group ofthree markers are formed which passively reflect the infrared radiation.This group of three markers is positioned by the surgeon on one of thefemoral or tibial portions of the joint, for example, on the lowerportion of the femur, in order to form a three-dimensional markingsystem which allows the camera to determine in conventional manner theprecise geometric location of one or more supplementary markers whichare positioned on a bone portion which can be moved relative to thereference system of the first three markers, for example, positioned onthe tibia. The surgeon is thus able, using appropriate calculationmeans, to determine, relative to the spatial reference system of thethree markers which are positioned on the femur, the precise position ofthe tibia and, in particular, the angle of flexion between the twobones, the spacing between these bones, the lateral and anteroposteriordisplacements and the relative rotations.

The operation of the tensioning device is explained below, in thecontext of positioning a prosthetic knee joint.

The surgeon has a sterile set of implants of different sizes, eachimplant conventionally comprising: a tibial component formed by a baseco-operating with a tibial rod in order to effectively seal the base ona cutting surface of the tibial plate, and having, for each type ofbase, a set of tibial plates, for example, of polyethylene, which can beattached to the base in order to provide a prosthetic tibial articularsurface; a femoral component comprising a distal end which co-operateswith a femoral rod which is intended for sealing in the femoralmedullary canal, and having a prosthetic trochlea component which isintended to be articulated to the tibial plate, this trochlea componentbeing either fixedly joined directly to the distal femoral end or, inother models, being able to be attached thereto, for example, with shimsbeing interposed from a set of shims of variable thicknesses; the tibialcomponent and the femoral component being connected or not in anarticular manner by a pivoting means.

After positioning the various infrared markers, as explained above, thenacquiring the anatomical forms of the relevant portions of the femur andthe tibia and obtaining the precise anatomical model of these forms anddimensions using the calculation means mentioned above, the surgeoncarries out, if necessary, a resection of the defective tibial plateand, using the movable marker, marks the position of this cutting plane.

Preferably, during the operation, the calculation means continuouslyprovide the values of the current flexion and the angle HKA.

The surgeon then places the knee in a state of flexion, for example, atapproximately 20°, and inserts the tensioning device 1 at the inner sideof the joint. The support surface 20A is placed in contact with, or atleast facing, the inner condyle of the femur, the support surface 20B isplaced in contact with, or at least facing, the outer condyle of thefemur and the support surfaces 24A and 24B are placed in contact withthe natural tibial plate or the one which is obtained after resection.More precisely, by way of example, for each condyle compartment, a firstinsert is positioned, the second insert is then positioned after havingdislocated the kneecap at the opposite side.

The force generation units 30A and 30B are activated so as to place theinner and outer compartments of the knee in a state of tension. For eachcompartment, a predetermined force is imposed, controlled by thepressure gauges 42A and 42B. The infrared markers allow the surgeon toverify that the spacing between the femoral and tibial portions of theknee joint is satisfactory. The simultaneous recording of the tensioningforce and the spacing distance allows better calculation of the optimumtension of the soft parts. In the case of unsatisfactory femorotibialalignment, he relaxes or tightens the appropriate ligaments in order tomove the femoral and tibial portions away from or towards each other.

Whilst holding the tensioning device 1 in position, the surgeon movesthe knee into different positions of flexion and repeats these samemeasurements. Owing in particular to the concave form and the slidingproperties of the condyle support surfaces 20A and 20B, the device 1 isstabilised relative to the knee during displacement. The surgeon bringsthe knee, for example, into the region of 0° of flexion, that is to say,into a state of extension. He continuously and intraoperatively verifiesthat, under the tension imposed by the support surfaces 20A, 20B, 24Aand 24B, the relative spacing of the femoral and tibial portions issatisfactory.

In other words, the dynamic examination of the length, that is to saythe tension, of the soft parts of the joint during the movements offlexion and extension allows the retraction diagnostics for these softparts, in particular posterior parts, to be carried out and thereforeallows the surgical actions for freeing or tightening these soft partsto be guided.

Advantageously, the integration of,the dynamic data on the one handallows the anatomical centres of condyle rotation to be determined.Knowledge of these is necessary in order to decide the best positioningof the femoral implant. If the prosthetic centre of condyle rotation isoffset or eccentric relative to the anatomical centre of rotation,stability, although correct at 0 and 90° of flexion, will be compromisedfor the intermediate angles, in particular at 45°, the soft parts beingeither too relaxed or too tight depending on whether the prostheticcentre is offset towards the front or the rear, or in the proximal or inthe distal direction. On the other hand, the femorotibial alignment inthe state of extension is measured correctly, the soft parts havingcorrect tension. The surgeon, assisted by the above-mentionedcalculation means, can measure this alignment in a dynamic mannerbetween 0 and 120°, or even 150° of flexion of the knee using the deviceaccording to the invention. In the case of unsatisfactory alignment,surgical actions for freeing the soft parts are carried out andcontrolled intraoperatively using the device according to the invention.

Furthermore, once the femoral articular prosthesis of the knee ispositioned, the device according to the invention allows the prostheticfemorotibial distances to be determined at a given pressure during theflexion/extension movements. The patient can thus be sure that thesurgical procedure will be carried out correctly.

Other uses of the tensioning device 1 can also be envisaged:

-   -   once the device has been positioned on the natural or prosthetic        joint, and is tensioned at a predetermined strength, the surgeon        is able to examine the variations in pressure around the        pressure value initially imposed, brought about by the movements        of the femoral and tibial portions of the joint; these        “response” variations are, for example, measured by the pressure        gauges 42A and 42B or the sensors connected to the servomotor;        the surgeon thus continuously verifies that the soft parts        behave in the anatomically anticipated manner during flexion of        the joint; advantageously, these measured values are transmitted        to the, above-mentioned calculation means which determine the        precise values of the tensions imposed by the soft parts; and    -   at the beginning, during or at the end of an operation, each        assembly 2A and 2B can be used independently in order to        determine the dynamic behaviour of each articular compartment;        in this manner, if the differences in behaviour of the        compartments deviate from the anticipated anatomical tolerances,        the surgeon carries out adjustments for freeing or tightening        the corresponding inner or outer ligament portions.

Furthermore, various arrangements and variants of the device describedin detail above can be envisaged:

-   -   the sliding rollers 12A and 12B can be replaced with a series of        juxtaposed ball bearings which are mounted so as to rotate        freely or a smooth surface having a low friction coefficient, in        order to allow the condyle surface to slide;    -   each force generation unit 30A and 30B can be replaced by a        servomotor which-has a specific and adjustable force and which        either allows a specific constant force to be generated, or, in        the case of “response” measurement of the pressures applied by        the femoral and tibial portions, allows the corresponding forces        to be measured, or by a connector which is formed from shape        memory metal;    -   the two tibial inserts 10A and 10B can be rigidly connected to        each other so as to form a single tibial insert, the femoral        inserts each being articulated relative to this single tibial        insert, or optionally rigidly connected to each other; and/or    -   each pair of articulated branches 4A, 4B can be replaced with        two arms which are substantially parallel with each other and        which are provided, at the distal end thereof, with        intra-articular inserts 8A and 10A, these two arms being able to        be moved relative to each other in accordance with a translation        movement which brings about the relative spacing of the        intra-articular inserts, for example, by means of a motorised        assembly having an endless screw, interposed between the two        arms.

The device according to the invention can also be used as a tensor forthe femoropatellar joint, the femoral insert being pressed against thetrochlea surface and the tibial insert against the inner face of thekneecap. It is also possible to make provision for the dimensions andshapes of the inserts to be adapted for this purpose by conferring, inparticular on the femoral insert, a convex form which complements thesurface of the trochlea, or to have sets of inserts which can beassembled and disassembled on the arms of the tensor.

1. Device for dynamic tensioning of a natural or prosthetic knee joint,with or without a tibial cut being carried out, of the type comprisingat least one femoral insert (8A) which has a condyle support surface(20A) for a femoral implant or bone; at least one tibial insert (10A)which has a support surface (24A) for a tibial plate for a tibialimplant or bone; and means (4A, 30A) for applying, between the femoraland tibial inserts, a distraction force of a predetermined strength,with or without the kneecap being in position, characterised in that itis constructed so as to allow rotation of the joint and comprises meansfor maintaining the knee in a state of tension during rotation, and thuscarrying out measurements for various angles of rotation.
 2. Deviceaccording to claim 1, characterised in that the condyle support surface(20A) is in the form of a dish.
 3. Device according to claim 1,characterised in that the condyle support surface (20A) is provided withsliding means (12A) for the femoral implant or bone when the knee jointis displaced.
 4. Device according to claim 3, characterised in that thesliding means comprise juxtaposed rollers (12A).
 5. Device according toclaim 3, characterised in that the sliding means comprise juxtaposedball-bearings.
 6. Device according to claim 1, characterised in that thecondyle support surface (20A) is substantially cylindrical, having anaxis (X-X) which is substantially transverse relative to the directionof distraction.
 7. Device according to claim 1, characterised in thatthe maximum thickness of each femoral insert (8A, 8B) and tibial insert(10A, 10B) is less than or equal to 2.5 mm.
 8. Device according to claim1, characterised in that a femoral insert (8A, 8B), and optionally atibial insert (10A, 10B), is/are provided for each inner and outercompartment of the knee joint.
 9. Device according to claim 1,characterised in that it comprises means for measuring the spacing ofthe condyle support surfaces (20A) and tibial plate support surfaces(24A), which means are capable of continuously measuring the spacingbetween the support surfaces when the knee joint is displaced. 10.Device according to claim 1, characterised in that it comprises means(42A) for measuring the distraction force between the femoral inserts(8A) and tibial inserts (10A), which means are capable of continuouslymeasuring the variation of the strength of the distraction force aroundthe predetermined strength thereof when the knee joint is displaced. 11.Device according to claim 1, characterised in that the means forapplying the distraction force comprise a force generation unit (30A)and a pair of branches (4A) which connect the generation unit to thefemoral inserts (8A) and tibial inserts (10A).
 12. Device according toclaim 2, characterised in that the condyle support surface (20A) isprovided with sliding means (12A) for the femoral implant or bone whenthe knee joint is displaced.
 13. Device according to claim 4,characterised in that the sliding means comprise juxtaposedball-bearings.
 14. Device according to claim 2, characterised in thatthe condyle support surface (20A) is substantially cylindrical, havingan axis (X-X) which is substantially transverse relative to thedirection of distraction.
 15. Device according to claim 3, characterisedin that the condyle support surface (20A) is substantially cylindrical,having an axis (X-X) which is substantially transverse relative to thedirection of distraction.
 16. Device according to claim 4, characterisedin that the condyle support surface (20A) is substantially cylindrical,having an axis (X-X) which is substantially transverse relative to thedirection of distraction.
 17. Device according to claim 5, characterisedin that the condyle support surface (20A) is substantially cylindrical,having an axis (X-X) which is substantially transverse relative to thedirection of distraction.
 18. Device according to claim 2, characterisedin that the maximum thickness of each femoral insert (8A, 8B) and tibialinsert (10A, 10B) is less than or equal to 2.5 mm.
 19. Device accordingto claim 3, characterised in that the maximum thickness of each femoralinsert (8A, 8B) and tibial insert (10A, 10B) is less than or equal to2.5 mm.
 20. Device according to claim 4, characterised in that themaximum thickness of each femoral insert (8A, 8B) and tibial insert(10A, 10B) is less than or equal to 2.5 mm.